Sharon Cui

Study of gate oxide traps in HfO2/AlGaN/GaN metal-oxide-semiconductor high-electron-mobility transistors by use of ac transconductance method

We introduce an ac-transconductance method to profile the gate oxide traps in a HfO2 gated AlGaN/GaN Metal-Oxide-Semiconductor High-Electron-Mobility Transistors (MOS-HEMTs) that can exchange carriers with metal gates, which in turn causes changes in analog and pulsed channel currents. The method extracts energy and spacial distributions of the oxide and interface traps under the gate from the frequency dependence of ac transconductance. We demonstrate the method using MOS-HEMTs with gate oxides that were annealed at different temperatures.

AC Transconductance Dispersion (ACGD): A Method to Profile Oxide Traps in MOSFETs Without Body Contact

We introduce an ac transconductance dispersion method (ACGD) to profile the oxide traps in an MOSFET without needing a body contact. The method extracts the spatial distribution of oxide traps from the frequency dependence of transconductance, which is attributed to charge trapping as modulated by an ac gate voltage. The results from this method have been verified by the use of the multifrequency charge pumping (MFCP) technique. In fact, this method complements the MFCP technique in terms of the trap depth that each method is capable of probing. We will demonstrate the method with InP passivated InGaAs substrates, along with electrically stressed Si N-MOSFETs.

Ga2O3(Gd2O3)∕Si3N4 dual-layer gate dielectric for InGaAs enhancement mode metal-oxide-semiconductor field-effect transistor with channel inversion

A dual-layer gate dielectric approach for application in III-V metal-oxide-semiconductor field-effect transistor (MOSFET) was studied by using ultrahigh vacuum deposited 7–8nm thick Ga2O3(Gd2O3) as the initial dielectric to unpin the surface Fermi level of In0.18Ga0.82As and then molecular-atomic deposition of ∼2–3nm thick Si3N4 as a second dielectric protecting Ga2O3(Gd2O3). The total equivalent oxide thickness achieved in this study is 5nm. We have demonstrated an enhancement mode In0.18Ga0.82As∕GaAs MOSFET with surface inverted n channel with drain current (Id) of 0.1mA for a gate length of 10μm and a gate width of 880μm at Vds=1V and Vg=4.5V.

Total-Ionizing-Dose Radiation Effects in AlGaN/GaN HEMTs and MOS-HEMTs

We have investigated the total ionizing dose (TID) radiation effects in AlGaN/GaN MOS-HEMTs as a function of dose and radiation bias, and compared them with conventional HEMTs. Under 10 keV X-ray irradiation, two distinct regimes of threshold voltage (V_th) shifts have been revealed: a rapid V_th shift at low doses for both HEMTs and MOS-HEMTs, and an additional V_th shift only found in MOS-HEMTs for doses up to at least 2 Mrad (SiO₂). The rapid V_th shift anneals quickly and is a strong function of layer material. We attribute this portion of the V_th shift to hole trapping in the AlGaN barrier layer. The V_th shift at high doses found only in MOS-HEMTs is attributed to hole trapping in the gate oxide. By comparing MOSFETs with HfO₂ and Al₂O₃ gate dielectrics that are annealed during processing at various temperatures, we find that 500^°C annealed HfO₂ shows the most promising TID response.

Improved AC conductance and Gray-Brown methods to characterize fast and slow traps in Ge metal–oxide–semiconductor capacitors

We use an improved AC conductance method and a modified Gray-Brown method to study fast interface traps and slow border traps in Ge-based MOS capacitors. The combined methods provide the corrected Fermi energy level (E) versus gate voltage (Vg) relationship, even in samples with high densities of traps that cause significant C-V distortion, the energy distribution of interface traps, their capture cross sections (σ), as well as slow border traps. A wide range of σ’s in p-type Ge is found, indicating that there is more than one type of interface trap near the Ge valence band edge. In contrast, a constant σ near the Ge conduction band edge is observed in n-type Ge. XPS results indicate that Ge suboxides near the interface are accountable for the detected slow border traps.

Effect of H on interface properties of Al2O3/In0.53Ga0.47As

We report that depositing Al2O3 on InGaAs in an H-containing ambient (e.g., in forming gas) results in significant reduction of interface-trap density and significantly suppressed frequency dispersion of accumulation capacitance. The results of the inelastic electron tunneling spectroscopy study reveal that strong trap features at the Al2O3/InGaAs interface in the InGaAs band gap are largely removed by depositing Al2O3 in an H-containing ambient. Transmission electron microscopy images and x-ray photoelectron spectroscopy data shed some light on the role of hydrogen in improving interface properties of the Al2O3/In0.53Ga0.47As gate stack.

A study of electrically active traps in AlGaN/GaN high electron mobility transistor

We have studied electron conduction mechanisms and the associated roles of the electrically active traps in the AlGaN layer of an AlGaN/GaN high electron mobility transistor structure. By fitting the temperature dependent I-V (Current-Voltage) curves to the Frenkel-Poole theory, we have identified two discrete trap energy levels. Multiple traces of I-V measurements and constant-current injection experiment all confirm that the main role of the traps in the AlGaN layer is to enhance the current flowing through the AlGaN barrier by trap-assisted electron conduction without causing electron trapping.

Electron tunneling spectroscopy study of electrically active traps in AlGaN/GaN high electron mobility transistors

We investigate the energy levels of electron traps in AlGaN/GaN high electron mobility transistors by the use of electron tunneling spectroscopy. Detailed analysis of a typical spectrum, obtained in a wide gate bias range and with both bias polarities, suggests the existence of electron traps both in the bulk of AlGaN and at the AlGaN/GaN interface. The energy levels of the electron traps have been determined to lie within a 0.5 eV band below the conduction band minimum of AlGaN, and there is strong evidence suggesting that these traps contribute to Frenkel-Poole conduction through the AlGaN barrier.

Effect of hydrogen on the chemical bonding and band structure at the Al2O3/In0.53Ga0.47As interface

Surface passivation of high mobility semiconductors such as InGaAs is a crucial bottleneck towards their integration in metal-oxide-semiconductor devices. The chemical structure and band offsets of InGaAs-Al2O3 with different passivations were investigated by x-ray photoelectron spectroscopy. Pre-deposition forming gas plasma treatment is shown to significantly improve the chemistry of S-passivated InGaAs surface, on which the Al2O3 is deposited by the molecular atomic deposition technique. Moreover, the change in the surface chemistry was found to correlate with a difference of 0.8 eV in the band offsets at the interface. This may offer insights on Fermi level pinning in such systems.

Total Ionizing Dose (TID) Effects in Extremely Scaled Ultra-Thin Channel Nanowire (NW) Gate-All-Around (GAA) InGaAs MOSFETs

InGaAs nanowire (NW) gate-all-around (GAA) MOSFETs exhibit superior radiation hardness compared to planar devices and FinFETs, benefitting from reduced gate-oxide electric fields. Applied gate bias during irradiation, channel thickness, and presence or absence of a forming gas anneal can strongly affect NW device radiation hardness. Low-frequency noise measurements are carried out to probe near-interfacial oxide-trap (border-trap) densities, and TCAD simulations are performed to assist in understanding the charge trapping in NW channel devices with high-k gate dielectrics. Optimized device structures exhibit high radiation tolerance.